Various air cleaners suction air through multiple filters. Some of them provide an alert signal for replacing the filters based solely on a predetermined time interval which has elapsed. The alert signal occurs regardless of whether the filters are actually clogged or degraded. This is because filters will clog at varying times depending on the amount of airborne particulates in the room. Other air cleaners with multiple filters use a device comprising a pressure gauge downstream of the filters which floats an indicator element to signal the need for replacement. However such a device fails to provide information as which filter should be replaced. It is possible that only one of the filters actually needs to be replaced. Accordingly, there is a need for an air cleaner and method which accounts for varying environmental conditions and which alerts the user as to when each one of the filters needs to be replaced.